Nhanulepisosteus mexicanus, Brito, Alvarado-Ortega & Meunier, 2017

Brito, Paulo M., Alvarado-Ortega, Jésus & Meunier, François J., 2022, Histological study of the oral teeth and their bony support in the Mexican Jurassic gar † Nhanulepisosteus mexicanus (Ginglymodii, Lepisosteidae), Cybium 46 (1), pp. 13-18 : 15-16

RESULTS

Tooth morphology

The dentary bears two rows of sharply pointed teeth: an outer one presenting numerous relatively small marginal teeth and an inner one with large and robust teeth ( Fig. 1 ). Within the inner row, the most anterior tooth is a hypertrophied fang, being at least five times bigger than the immediately posterior teeth (see Brito et al., 2017: figs 2C, D). The anterior coronoid is a medially curved, anteriorly broad bone that will suture with its antimerous rostrally. It presents small tiny teeth, similar in size to those of the outer row of the dentary.

Contrary to other lepisosteid teeth presenting plicidentine, in where the enamel grooves and striations of the tooth shaft are wider at the base of the tooth (Peyer, 1968; Schultze, 1969; Grande, 2010; Meunier and Brito, 2017), the teeth of † Nhanulepisosteus have very fine external grooves and striations distributed over almost the entire length of the teeth ( Fig. 1 ). The acrodine cap also seems to be less important.

Tooth histological organization

The sections reveal that there are three types of teeth according to their height, apart from the caniniform fang. They are:

1) The pointed teeth, clearly ridged externally ( Fig. 3 B-E). At their base, the teeth show true pleats of the enamel and dentine layers ( Fig. 3B ). Yet, upper this basal morphology, pleats are replaced by crests due to more or less thickening of the enamel layer ( Fig. 3 C-E). Under the enamel layer, there is a thick layer of dentine crossed by parallel odontoblastic canalicles ( Fig. 3E ). In the pulp cavity dentine canals are seen ( Fig. 3B, C ); they represent a plicidentine organization. This plicidentine occupies the whole pulp cavity ( Fig. 3 A-C), excepted in its upper part ( Fig. 3D ). The enameloid does not penetrate inside the dentine plies contrary to extant Lepisosteidae ( Schultze, 1969; Meunier and Brito, 2017). The dentine that fills the pulp cavity is constituted of vascular canals surrounded by dentinal tissue that is crossed by orthogonal odontoblastic canalicles ( Fig. 3 C-E); so the present dentine is true osteodentine. We note that † Atractosteus africanus (Late Cretaceous) and † A. occidentalis (Late Cretaceous) also show plies of the pulpar walls of their teeth ( Wiley, 1976: respectively fig. 59 and 38c).

2) Some mid-sized teeth present on the jaws ( Fig. 4 ). They also have an external ridged enamel layer ( Fig. 4 ) that overlays the dentine layer, but the dentine does not show plies in the pulp cavity that is totally empty ( Fig. 4 ).

3) The pharyngeal bony plates with numerous small teeth ( Fig. 5A ), some of them being fixed on the bone ( Fig. 5B ). These teeth are formed of a dentine cone with a pulp cavity that looks empty, without plies ( Fig. 5A, B ). Their external surface is not ridged but smooth ( Fig. 5A ).

Bone histology

The base of the teeth is fused to the dentary plate by attachment bone ( Figs 3A , 6A). The supporting bone is essentially constituted of vascular secondary bone with little areas of primary bone ( Figs 3A, B , 6B-E). The breach aspect of bony tissue is due to the presence of numerous secondary bone osteones ( Fig. 6 B-D) that characterize remodelling processes. This remodelling is directly linked with a regular and frequent replacement of teeth, and probably linked to the large size of teeth (see Brito et al., 2017: fig. 2C, D) that produces large scars when they fall. These impressive teeth can be considered as fangs, and so indicate a predatory diet for † Nhanulepisosteus like the extant Lepisosteidae ( Lagler et al., 1942; Porter and Motta, 2004). Primary and secondary bony tissues show star-shaped osteocytes ( Fig. 6E ), and they are crossed by canalicles, the so-called canalicles of Williamson ( Fig. 6C, E ). Some of them can dichotomise. The presence of canalicles of Williamson is perfectly suited to the identification of † Nhanulepisosteus in the Lepisosteidae family.

Figures

Figure 1. – †Nhanulepisosteus mexicanus: External view of the fossil sample (IGM 4901) showing four caniniform teeth with their apical cap of acrodine (arrows). Two small teeth are seen between the fangs (arrowheads). Scale bar = 1 mm.

Figure 3. – †Nhanulepisosteus mexicanus: Histological organization of the fangs (natural transmitted light). A: Axial section of a tooth. The pulp cavity (pc) is totally occupied by axial trabeculae of dentine (de) alternating with pulp spaces. The tooth is fixed on a basement of reconstructed bone (bo). vc = vascular cavities. B-D: Three cross-sections of different levels of a tooth. B: The section is localized at the tooth base; it shows a ridged crown of orthoden- tine surrounded by a thin layer of enameloid (en). The pulp cavity is occupied by vascularized dentine. On the bottom of the fig, the bone of attachment (bo). C: The section crosses the mid part of the pulp cavity. The tooth is constituted of a crown of orthodentine (de) surrounded by a layer of enamel (en) showing external crests. The pulp cavity is totally occupied by vascular- ized dentine. D: The section is in the higher part of the pulp cavity. The tooth is constituted of a crown of orthodentine (de) surrounded by the crenulated enamel (en). We can see the pulp cav- ity which is reduced. E: Cross-section in the mid part of the tooth. Details of the dentine show- ing the peripheral orthodentine layer (white asterisks), with its ondontoblastic canalicles and the vascular canals fit together. Each canal is surrounded by orthodentine (black arrow-head). The enameloid layer is crenulated (white arrow-heads). A: IGM 4902-5; B: IGM 4902-3; C,E: IGM 4902-2; D: IGM 4902-1. Scale bars:A, B, C = 200 μm; D = 100 μm; E = 50 μm.

Figure 4. – †Nhanulepisosteus mexicanus. A-B: IGM 4902-5. Cross-section (respectively natural and polarized transmitted light) of a small tooth. We can show a crown of dentine (de), with an external crenulated layer of enameloid (arrow-heads). The pulp cavity (pc) is empty and its wall is regular. Scale bar = 100 μm.

Figure 5. – †Nhanulepisosteus mexicanus: A: Pharyngeal dentigerous plate. We can see cross (white and black arrow-heads) and axial (white and black arrows) sections of minute teeth. We can also see a section of a scale (sc) (bo = bony tissue). B: Axial section of a pharyngeal tooth (white arrow) and its supporting bony plate (bp). A: IGM 4902-4; B: IGM 4902-5. Scale bars: A = 200 μm; B = 100 μm.

Figure 6. – †Nhanulepisosteus mexi- canus: Histological organization of the teeth supporting bone. A: Detail of the basal contact of a tooth (pc) and its supporting bone (bo) with vascu- lar canals or cavities (vc). The white arrows point to osteons. B: Detail of the dentigerous plate showing lamel- lar bone and numerous cementing lines (black arrow-heads) that character- ized an intensive remodeling of bone tissue below the tooth. We also show canalicles of Williamson (arrows). The asterisks point to secondary osteons. C-D: Detail of the secondary osteon (**) of Fig. 6B (respectively natural and polarized transmitted light). The secondary bone of the osteons is lamel- lar bone (black asterisks). The arrow- heads point to cementing line. We also show canalicles of Williamson (black arrows). E: Detail of the secondary osteon (*) of Fig. 6B showing canal- iculi of Williamson (black arrows) and osteocytes (black arrowheads). A: IGM 4902-4; B, C, D, E: IGM 4902- 5. Scale bars: A = 200 μm; B = 100 μm; C, D = 50 μm; E = 25 μm.